1. Field of the Invention
The present invention relates in general to automotive brake systems and particularly to a brake pressure control unit of the deceleration-responsive type for use in an automotive brake system.
2. Description of the Prior Art
During braking or deceleration of a vehicle, a portion of the weight borne by the rear wheels is transferred to the front wheels. For this reason, the rear wheels are more likely to be locked than the front wheels if the front and rear brakes are pressurized equally. The premature locking of the rear wheels can result in a so-called dangerous skidding causing the driver to lose control of the vehicle. For avoiding this dangerous locking of the rear wheels, it has been practiced to dispose a brake pressure control valve or unit in the rear brake systems for the purpose of controlling the hydraulic or fluid pressure applied to the rear wheel brake cylinders.
A so-called proportioning valve or limiting valve is one of the well-known examples of this kind of control valve and which is adapted to effect such a control that the rear wheel brake cylinder pressure increases more gently than the front wheel brake cylinder pressure after reaching to a predetermined pressure, generally called a critical pressure or change-over pressure. Such a control valve is constructed so that the change-over pressure is maintained constant and thus effects a constant distribution of the braking force irrespective of the variation of the load on the vehicle.
As is known, however, an ideal distribution of the braking force that causes the front and rear wheels to be locked simultaneously varies with the variation of the load on the vehicle. In order to attain such an ideal distribution of the braking force, it is therefore necessary for the brake pressure control valve or unit to be constructed such that the change-over pressure is variable depending upon the variation of the load on the vehicle, viz., the change-over pressure increases with the increase of the load on the vehicle.
A brake pressure control unit of the deceleration-responsive type was proposed for attaining this end, which is provided with a storing chamber to which a master cylinder pressure is supplied to compress or decompress a spring of a control valve for thereby increasing or reducing the change-over pressure that is determined by the force of the control valve spring. The master cylinder pressure supplied to the storing chamber is sealingly stored therein by the closure of a deceleration-responsive valve in response to a vehicle deceleration at a rate exceeding a predetermined value. By the closure of the deceleration-responsive valve, the storing chamber pressure is thus maintained constant irrespective of the further increase of the master cylinder pressure, maintaining the force of the control valve spring and therefore the change-over pressure constant. From this time onward, the further increase of the master cylinder pressure causes the rear wheel brake cylinder pressure to increase more gently than the front brake cylinder pressure. The change-over pressure thus can be varied with the increase of the load on the vehicle since the master cylinder pressure increases with the increase of the load on the vehicle.
This deceleration-responsive type brake pressure control unit is intended to effect such a control of the rear wheel brake cylinder pressure as is shown in FIG. 5. That is, when the vehicle is empty, such a distribution of the braking force that is represented by the line a - b - c (change-over pressure Ps.sub.1) is obtained. On the other hand, when the vehicle is in a certain loaded condition, such a distribution of the braking force that is represented by the line a - b' - c' (change-over pressure Ps.sub.2) is obtained.
However, this brake pressure control valve unit cannot provide such an intended control of the rear wheel brake cylinder pressure. That is, when the master cylinder pressure is increased at a rate as indicated by .alpha. in FIG. 6, the resulting rate of vehicle deceleration is retarded as indicated by .beta.. Thus, in the case of the above-mentioned brake pressure control valve unit wherein the storing chamber is adapted to be supplied with the master cylinder pressure as it is, the fluid pressure sealingly stored in the storing chamber by the closure of the deceleration-responsive valve does not correspond to the loaded condition of the vehicle but becomes too high. For this sake, the change-over pressure actually effected by the above-mentioned deceleration-responsive type brake pressure control unit becomes higher than the intended pressure as for example indicated by Ps.sub.1 and Ps.sub.2 in FIG. 5 and are displaced from the intended point b or b' to the point d or e as shown in FIG. 5.
To solve this problem, it has been proposed to dispose a pressure-reducing valve in the fluid passage leading to the storing chamber, intending to supply to the storing chamber such a fluid pressure that is smaller than the master cylinder pressure by a certain constant amount and thereby to retard the increase of the storing chamber pressure relative to the master cylinder pressure by the amount corresponding to the time t by which the vehicle deceleration is retarded to occur after application of the brake.
With this countermeasure, the intended result is obtained only when the rate of increase of the master cylinder pressure is such particular one as represented by .alpha. in FIG. 6 and cannot be obtained when the rate of increase of the master cylinder pressure is slower as represented by .alpha.' in FIG. 7 or faster as presented by .alpha." in FIG. 8. This is because the vehicle deceleration is retarded to occur by a constant time t for any rate of increase of the master cylinder pressure, as represented by .beta.' and .beta." in FIGS. 7 and 8, while the above-mentioned pressure-reducing valve is adapted to supply to the storing chamber such a pressure (.delta.' or .delta." in FIGS. 7 or 8) that is smaller than the master cylinder pressure (.alpha.' or .alpha.' in FIGS. 7 or 8) by a constant amount .gamma.. For this sake, the time by which the increase of the storing chamber pressure is retarded relative to the master cylinder pressure cannot always correspond to the time by which the vehicle deceleration is retarded to occur after application of the brake. By the use of such a pressure-reducing valve, it has therefore been impossible to make the storing chamber pressure precisely correspond to the loaded condition of the vehicle under any vehicle deceleration rate and for this reason it has been impossible to attain a proper distribution of the braking force at all times under any operating condition of the vehicle.